1. Field of the Invention
The present invention relates to a method of bending a metallic pipe such as a square pipe, a small-diameter pipe, a multiple pipe, or the like.
2. Description of Related Art
An exhaust pipe for a motorcycle is subjected to a surface treatment to aesthetically improve the external appearance and has a double-pipe or multiple-pipe construction to prevent the surface-treated layer from deteriorating in color due to the heat of the exhaust gas.
Conventionally, in bending this kind of multiple pipe, the bending work is carried out after filling the clearance between each of the constituent metallic pipes with various kinds of filling materials including metals of low melting point, particles such as sand, or the like in order to prevent the inner and outer pipes from buckling as well as to keep the clearance between each of the constituent metallic pipes to a predetermined distance.
However, when a metal of low melting point is used as the filling material, it is difficult to discharge the metal out of the clearance after the bending work has been finished. Post-treatment is thus troublesome and, as a result, the machining or working cost becomes high. When particles such as sand are used, they are partially agglomerated, or solidified into larger lumps, due to the pressure at the time of bending and therefore are difficult to be discharged. Further, after the bending work has been finished, there will sometimes remain irregularities on the surface of the metallic pipe, thereby impairing the external appearance.
In order to cope with the above-described problems, there is used wax which is easy to be discharged after the bending work has been finished. However, there also exists a problem in that, due to insufficient hardness of the wax, the metallic pipe is likely to be flattened, or get out of roundness, or the inner pipe is likely to become eccentric relative to the outer pipe. There is also proposed a method of bending a multiple metallic pipe after freezing the water which is sealed into the clearance between the constituent metallic pipes. This method necessarily requires a large-scale equipment such as a freezing apparatus for freezing the water as well as a low-temperature working space for carrying out the bending work while maintaining the frozen condition of the water. Further, there is another problem in this method in that, under such low-temperature conditions, the bending work becomes difficult due to the low-temperature brittleness of the metal.
On the other hand, in bending a single layer metallic pipe, the pipe is prevented from being flattened, or got out of roundness, by inserting therein a core metallic member which is arranged in the form of a rosary or a string of beads. This method has a problem in that the adjustment of the core metallic member is time-consuming. Further, since the core metallic member cannot be inserted into a pipe of small diameter, there is sometimes used the above-described method of bending the metallic pipe by filling it with a filling material including metals of low melting point, particles such as sand, or the like. There also arises a similar problem as described above in that the filling material is hard to be discharged after the bending work has been finished.
In view of the above disadvantages, the present invention has an object of providing a method of bending a metallic pipe in which the bending work of the metallic pipe can be carried out at a room temperature without causing the metallic pipe to get out of roundness or causing an inner pipe in a multiple pipe to become eccentric relative to the outer pipe and in which the post-treatment can be easily done.
According to the present invention, the foregoing and other objects are attained by a method of bending a metallic pipe comprising the steps of: filling, in a molten state, a filling material comprising essentially a fatty acid into the metallic pipe; bending, at a room temperature, the metallic pipe in a condition in which the filling material is solidified; and melting and discharging the filling material.
According to another aspect of the present invention, there is provided a method of bending a metallic pipe constructed in a form of a multiple pipe having a plurality of constituent metallic pipes of different diameters, one being disposed inside the other or another, said method comprising the steps of: filling, in a molten state, a filling material comprising essentially a fatty acid into at least the clearance between the constituent metallic pipes; bending, at a room temperature, the multiple pipe in a condition in which the filling material is solidified; and melting and discharging the filling material.
Preferably, the filling material is a mixture of the fatty acid and a powdery additive, depending on the kind of metallic pipe to be bent or machining conditions. Further, the filling material may comprise essentially a composite fatty acid which is a mixture of a plurality of fatty acids. When the composite fatty acid is used, it may be a mixture in a mixing ratio preferably of from 80 : 20 by weight to 10 : 90 by weight of stearic acid and palmitic acid.
Since the fatty acid solidifies at a room temperature and has a higher hardness when solidified than that of wax or the like, it enables the metallic pipe to be bent at a room temperature by restricting the flattening, or getting out of roundness, of the metallic pipe or the eccentricity of the inner pipe relative to the outer pipe in the multiple pipe. By mixing the fatty acid with the powdery additive, the hardness at the time of solidification increases with the result that the flattening, or getting out of roundness, of the metallic pipe or the eccentricity of the inner pipe relative to the outer pipe can be minimized.
Since the fatty acid becomes molten at 40-80.degree. C., the fatty acid can easily be discharged after the bending work has been finished and thus the post-treatment becomes easy.
The composite fatty acid which is a mixture in a mixing ratio of from 80 : 20 by weight to 10 : 90 by weight of stearic acid and palmitic acid is low in its melting point and has a high hardness when solidified. It is therefore suitable as the filling material.